For various reasons related in particular to better comfort of use (softness, emollience and others), current cosmetic or dermatological compositions are generally provided in the form of an emulsion of the oil-in-water type (oily phase dispersed in an aqueous phase constituting the continuous phase) or an emulsion of the water-in-oil type (aqueous phase dispersed in an oily phase constituting a continuous phase). Oil-in-water (O/W) emulsions are the most in demand in the field of cosmetics owing to the fact that they contribute to the skin, on application, a softer, less greasy and lighter feel than water-in-oil (W/O) emulsion systems.
Oil-in-water emulsions are generally stabilized by emulsifying surfactants of the oil-in water type which, by virtue of their amphiphilic structure, become positioned at the oily phase/aqueous phase interface and thus stabilize the dispersed oil droplets. Despite the presence of emulsifiers, the emulsions may have a tendency to phase separate (separation of the aqueous and oily phases with release of oil). Increasing the content of emulsifier generally makes it possible to improve the stability of the emulsion. However, the presence of emulsifiers at high concentrations leads to problems of discomfort of the composition obtained when used cosmetically, such as, for example, a rough feel or a sticky or tacky feel, and to irritation problems with respect to the skin, eyes and scalp, as a result of a large amount of surfactants.
In an attempt to solve the problems of stability of conventional O/W emulsions, “ultrafine” O/W emulsions have been proposed in which the mean size of the globules constituting the oily phase is within highly specific limits, namely between 50 and 1000 nm. Such ultrafine O/W emulsions are generally obtained according to a phase-inversion emulsification technique.
This technique is, in its principle, well known to a person skilled in the art and is described in particular in the articles “Phase Inversion Emulsification”, by Th. Forster et al., which appeared in Cosmetics & Toiletries, Vol. 106, December 1991, pp. 49–52; and “Application of the phase-inversion-temperature method to the emulsification of cosmetics” by T. Mitsui et al., which appeared in American Cosmetics and Perfumery, Vol. 87, December 1972. The principle of this technique is as follows: a W/O emulsion (introduction of the aqueous phase into the oily phase) is prepared at a temperature which is greater than the phase inversion temperature (PIT) of the system, that is to say the temperature at which the equilibrium between the hydrophilic and lipophilic properties of the emulsifier or emulsifiers employed is reached; at high temperature, that is to say greater than the phase inversion temperature (>PIT), the emulsion is of water-in-oil type and, during its cooling, this emulsion inverts at the phase inversion temperature to become an emulsion of oil-in-water type, this inversion being achieved because the emulsion is previously passed through a microemulsion state.
These ultrafine emulsions are generally extremely fluid. They have a bluish appearance and may be translucent. However, these emulsions are precarious and still present a number of stability problems. In particular, phenomena of creaming and/or of phase separation are observed after several freezing cycles, as well as the appearance of whorls and/or deposits which are difficult to identify.
One of the parameters strongly influencing the stability of O/W emulsions is the size of the dispersed oil droplets, which size is related to the surface tension between the noncontinuous (oily) phase and the continuous (aqueous) phase. The smaller the size of the oil droplets, the lower the surface tension and the greater the stability of the emulsion.
It is known to gel this type of emulsion for the purposes of improving its stability and of widening the range of viscosities, so as to be able to use it in any cosmetic and dermatological field, and of diversifying its textures, in order to adapt them to any skin type.
Unfortunately, the gelling agents commonly used in cosmetics generally have a poor performance in this type of composition. Furthermore, some gelling agents are incompatible with this type of emulsion. Generally, conventional gelling agents do not make it possible to homogeneously thicken emulsions obtained by phase inversion and to stabilize them in different viscosity ranges. In addition, owing to the fact that these gelling agents thicken the compositions, they do not make it possible to obtain compositions having the fluidity desired for certain uses, for example for being used in the form of a spray.
The need thus remains for ultrafine emulsions which can be very fluid while having satisfactory stability. The term “satisfactory stability” is understood to mean that the oil globules remain well distributed in the continuous aqueous phase without phase separation or whorling or deposition or creaming or any other emulsion instability phenomenon occurring.